The 80% aqueous acetone extracts from the fruit (50 mg/kg/d) and seeds (12.5 and 25 mg/kg/d) of Rosa canina L., but not from the pericarps, were found to show substantial inhibitory effect on the gain of body weight and/or weight of visceral fat without affecting food intake in mice for 2 weeks after administration of the extracts. With regard to the active constituents, the principal constituent, trans-tiliroside (0.1-10 mg/kg/d), potently inhibited the gain of body weight, especially visceral fat weight, and significantly reduced blood glucose levels after glucose loading (1 g/kg, ip) in mice. On the other hand, kaempferol and p-coumaric acid lacked such effect and kaempferol 3-O-beta-D-glucopyranoside tended to reduce the gain of body weight and visceral fat weight, but not significantly, at a dose of 10 mg/kg/d. These results indicate the importance of both kaempferol 3-O-beta-D-glucopyranoside and p-coumaroyl moieties for anti-obese effects. Furthermore, a single oral administration of trans-tiliroside at a dose of 10 mg/kg increased the expression of PPAR-alpha mRNA of liver tissue in mice.